This invention relates to body scanning apparatus and, more particularly, to apparatus for determining the contour of a scanned body in the plane of rotation of the scan head. In its most immediate sense, this invention relates to gamma cameras such are used to form tomographic images of a patient during a nuclear medicine study.
In single photon emission computed tomography (SPECT) systems of the transaxial rotational camera type, a scanning gamma camera head rotates around the region (head, heart) of the patient to be imaged. This rotation is in a plane generally orthogonal to the cranial-caudal axis of the patient and results in the imaging of a cross-sectional slice of the patient's body. Ideally, it is desirable for the camera head to be as close to the body as possible, because this results in increased sensitivity and, consequently, a better image (if the length of the study is held constant) or a shorter study (if the image quality is held constant).
It is known that the orbit of the camera head can be made noncircular with respect to the patient to decrease the average distance between the camera head and the patient. One device for producing such a noncircular orbit is disclosed in U.S. Pat. No. 4,503,331. In this, an effectively elliptical orbit is achieved by combining a circular rotation of the camera head with a linear movement between the head and the patient.
Devices of this type are not entirely satisfactory because they must be adjusted to match the head orbit with the particular patient to be examined. This adjustment is time-consuming and, consequently, the use of such devices reduces patient throughput and consequently increases study costs.
It is alternatively possible, as is shown in U.S. Pat. No. 4,593,189, to provide a camera head with a proximity detector and to urge the head towards the patient while rotating it around the patient. When the head gets too close to the patient, the proximity detector is triggered and the head is moved away from the patient. This solution has not been implemented; one reason may be that it would be difficult to change collimators on a camera head which is provided with such a device. It would also be difficult to use this solution in a multi-headed camera because it would be highly expensive to provide each head with a proximity detector.
It is therefore a primary object of this invention to provide an apparatus which automatically determines the contour of the body in the imaging plane (thereby eliminating the need for throughput-reducing manual adjustment) without mounting a proximity detector on the rotating camera head.
Another object of the invention is to provide such an apparatus which can be used to implement noncircular orbits in scintillation cameras of the multi-head type.
Another object is, in general, to improve on methods and apparatus of this general type.